llvmpipe: Cope with null Vertex element cso.

llvmpipe: Cope with null Vertex element cso.

CSO can often be null.

For example:
1. at initialization
2. using an util module (u_blit) right after initialization (it will push
   state and pop the previous null state)
3. at shutdown time (state shouldn't be bound when being destroyed)

Glean was hitting 2.